This invention relates to heat treatment of particulate materials and, more particularly, but not exclusively, to the calcining of ores, precipitates, concentrates and residues. In particular, the invention is applicable to fluid-bed calcination of phosphates, limestone, silicates of aluminum, alkaline earth minerals and sludges, and the like.
Heat treatment at less than melting temperatures, e.g., calcining, of particulate materials in a vertically-oriented fluidized bed, is well known. Such a fluidized bed is formed when a fluidized gas flows upwardly through a bed of suitably sized solid particles at a velocity sufficiently high to buoy the particles, to overcome the influence of gravity thereon, and to impart thereto an appearance of great turbulence. In some instances, the fluidizing gas used to form the fluidized bed can be air. More advantageously, the fluidized gas contains heat-generating components, i.e., fuel, which provides the heat for accomplishing the calcining in the fluidized bed.
The heating of the fluidized bed can be achieved either by using hot combustion gases as the fluidizing gas, or by actually burning a fuel within the fluidized bed itself. Among the known advantages for the latter method are the elimination of many restrictions on the type of materials that are available for fabrication of the fluidization grid (distribution plate). For example, when the heat is generated by burning fuel in the fluidized bed, the grid need not withstand high temperatures. In addition, in-bed combustion of fuel provides a more even temperature distribution within the fluidized bed, thereby accomplishing more uniform heat treatment.
The combination of the fluid bed technology and flash calcining has heretofore been employed when treating alumina to increase equipment capacity. This technology has not previously been adopted to the treatment of phosphate rock, limestone and other materials that are so heat sensitive that excessive temperatures (or insufficient residence time) would tend to result in unsatisfactory calcined products. Furthermore, conventional units, utilizing the fluidized bed approach for calcining, accomplish all the heat treatment in one compartment with various other compartments provided merely to assist in the recovery of heat. Because calcination is so strongly endothermic, heat must be supplied both to maintain the reaction temperature and to satisfy the heat requirements of the reaction. As a consequence, conventional systems employ large excesses of air, frequently more than 30% excess, with a resulting loss in thermal efficiency. Still further, such conventional vertical fluid bed systems are usually limited to one type of fuel and the inherent minimum fluid bed temperature resulting therefrom.